1. Field of the Invention
The present invention relates to an air-conditioning system and a refrigeration cycle control system, for controlling the supercooling degree of a liquid refrigerant of a condenser to a predetermined target value by controlling an electric expansion valve of a refrigeration cycle, which, in addition to the condenser and the electric expansion valve, includes a compressor and an evaporator.
2. Description of the Related Art
Japanese Unexamined Patent Publication (Kokai) No. 9-14780 discloses a system in which the supercooling degree in the condenser is controlled to a proper value in order to control the efficiency of the refrigeration cycle to a maximum. Specifically, first, a target supercooling degree providing a target value of the supercooling degree is determined in accordance with the atmospheric temperature or the like. A supercooling degree is calculated by an air-conditioning control unit from the pressure detected by a high-pressure sensor arranged on the outlet of the compressor and the temperature detected by a refrigerant temperature sensor arranged on the refrigerant outlet of the condenser. The valve opening degree of the electric expansion valve is controlled by the air-conditioning control unit in such a manner that the calculated supercooling degree assumes the target supercooling degree. Specifically, in the case where the supercooling degree is increased, the valve opening degree is decreased, so that the high pressure is increased to increase the liquid refrigerant in the condenser. In the case where the supercooling degree is decreased, on the other hand, the valve opening degree is increased to reduce the high pressure for a reduced liquid refrigerant in the condenser.
In the above-mentioned conventional system, a cooling control valve and a heating control valve are provided as the electric expansion valve in a refrigeration cycle of a heat pump type in which the refrigerant flow is switched between the cooling and the heating modes. When the cooling mode is started, for example, the valve opening degree of the cooling control valve is maximized, so that an abnormal increase in the high pressure is prevented at the time of starting the refrigeration cycle. At the same time, the refrigerant circulation amount at the time of starting is secured in an effort to improve the starting characteristic of the refrigeration cycle.
Also, in the conventional system, at the time of cooling, for example, a target temperature of the air passed through the evaporator is calculated in accordance with the set position of a blowing temperature setting lever, and the rotational speed of the compressor is controlled to attain the target temperature.
The high pressure sensor and the refrigerant temperature sensor, however, are varied in accuracy from one product to another. The calculated, i.e. theoretical supercooling degree described above, therefore, sometimes develops a considerable error.
For this reason, some automotive vehicles pose the problem that the calculated supercooling degree increases beyond the actual supercooling degree so that the actual refrigeration cycle is operated with such a low efficiency as to have a dryness of zero or less in supercooling degree. In other cases, the calculated supercooling degree becomes smaller than the actual supercooling degree, in which case the actual refrigeration cycle is deteriorated to low efficiency due to the excessively large supercooling degree.